Just a quick summary of the known characters we have discovered are to be in book 6. And they are belters!
Shadrak Meduson was Commander of the Iron Hands Legion's 10th Company - Sorrgol Clan.
Later having missed the rendezvous for the disasterous assault on Istvaan V, he became Local acting Commander of Loyalist Imperial resistance in the Segmentum Obscurus and led loyalist forces in the Battle of Dwell. During the battle Shadrak nearly assassinated Horus, Fulgrim, and Mortarion as they met on the Death Guard ship Silence with an ambush of Fire Raptors. Shadrak's second in command was Bion Henricos who was Sergeant in the 10th Company of the Sorrgol Clan.
Tybalt Marr and Verulam Moy were Captains of the Luna Wolves Space Marine Legion, commanding the 18th and the 19th Companies, respectively.
Both were 'Sons of Horus', the term for those Luna Wolves Marines who looked facially similar to their Primarch Horus. While their similarity in appearance to Horus was notable, the similarity between each other was so striking that they were considered almost twins by their fellows. This carried on into their respective nicknames, 'The Either' and 'The Or', due to the belief that they were so hard to tell apart they may as well be interchangeable. Tybalt Marr was referred to as The Either while Verulam Moy was called 'The Or.'
It was on Davin's moon that Moy met his fate. A member of the warrior-lodge and a competent file officer, Moy had yet to distinguish himself as one of his legion's elite. His time appeared to have arrived when he was selected by the Warmaster to lead his personal security unit into the Glory of Terra, the crashed flagship of Eugen Temba. Shortly afterwards the Marines were largely separated from each other and their commander, and a lone Verulam Moy was the first to locate Temba. Temba attempted to sway Moy to the embrace of Chaos, but the Marine Captain resisted the temptation. A brief battle then took place, in which Moy was killed by the traitorous Temba.
It was Horus who introduced Marr to the Lodges.
During the Heresy his character really gets developed, so much so that it sounds as though Horus tasked him with hunting down the Splintered loyalist forces.
Nathaniel Garro was a man who risked everything to bring word of Horus' Betrayal at Isstvan III on board the starship Eisenstein. He is the main figurehead of the Malcador's Knight Errants.
Look for a day one cut and shunt from me to make him a Death Guard Captain for our Great Crusade.
An extremely powerful psyker, Rubio was originally a Codicier of the Ultramarines Legion, before the Emperor's Decree after the Council of Nikaea banned the Space Marines' use of psykers. Rubio resented the Decree, but honoured his oath to the Emperor and surrendered his Psychic hood and Force sword, and returned to the lines as an ordinary Battle brother.
He was collected on Calth by Garro.
Now I would like to open the doors to speculation on others after this.
Sharrowkyn would be a great shout, but I would be sad if Wayland did not make a joint appearance. I am pretty sure we might only see one of each Legion characters :(
It could possibly be the only other opportunity to bring Numeon in, given the events of Deathfire. It is my sworn belief that we should see every first captain of the Legions.
There will also be a Blackshirld character but I have found no other info on this.
Hope to hear more about these chaps eventually, but to me they are as Blackshield as you can be.
If not I will make them :)